On 23/09/22 11:21, Paul Koning via cctalk wrote:
Cray did? I didn't know that. It first appeared in the CDC 6600, and yes, according to rumor at the request of NSA. I can imagine it being used for statistical analysis of character patterns
Yep. Pop count is essentially the Hamming Weight, which is a measure of information content and for a hunk of text can serve as a hash to drive further analysis.
A non-classified example of its use is in the PLATO system for "fuzzy string matching". PLATO needed to be able to recognize student answers that were correct but misspelled; it would do that, roughly speaking, by taking the difference of the expected string and the actual input and running population count on that. A difference of less than n bits would be defined as a misspelled match.
This is a use of Hamming Distance (or a riff on it). XOR two identical length binary strings, a pop count on the result tells you how "far apart" they are.
-- Christian Kennedy, Ph.D. ch...@mainecoon.com AF6AP | DB00000692 | PG00029419 http://www.mainecoon.com PGP KeyID 108DAB97 PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97 "Mr. McKittrick, after careful consideration…"
